DESCRIPTION: This study is designed as a multi-site randomized clinical trial to compare two sets of restorative dental materials, amalgam vs. composite for one and two-surface restorations and stainless steel crowns for larger restorations. Four specific aims are outlined. (The first three are similar to those described in the RFA, and the fourth one is unique.) 1. To investigate the potential psychological/behavioral, neurological, renal or immunological impairments or dysfunction in children that can be attributed to exposure to dental amalgam restorations. 2. To investigate the degree to which mercury concentrations in urine, blood, hair, and in the mouth differ in children with and without amalgam restorations. 3. To investigate the change in mercury concentrations in the urine, blood, hair, and in the mouth over time, beginning with the placement of the restorations. 4. To investigate the relationship between dose, as measured by number of amalgam surfaces, various biomarkers of mercury exposure and possible functions thereof, and response defined by the presence of observed health effects. About 1,000 participating English-speaking children who are 6-8 years old at baseline will be followed for five years. Additional inclusion criteria are for children to have three or more carious teeth, have no prior amalgam fillings, no clinical evidence of existing psychological, behavioral, neurologic, immunosuppressive disorders or disorders affecting kidney function, and no evidence of high seafood consumption. The population will be recruited from clinical centers in Maine affiliated with the Center for Community Dental Health and four locations in the Boston area. All of these clinics treat underserved populations who have a backlog of treatment needs. All children will receive dental care at these clinics by dentists trained in study protocol. At each site, prior to randomization, children will be stratified by caries level, either needing 3-4 or 5 or more restorations. The primary outcome measure is mean change in IQ during the five-year study period. Outcome variables include a battery of neuropsychological assessments with most repeated annually. Renal function will be assessed by urinalysis of urinary porphyrin profiles, gamma-glutamyl transpeptidase, and albumin levels. Mercury exposure will be assessed by determining the number of amalgam surfaces, surface area of amalgam, the release rate of mercury vapor from amalgams using a procedure adapted by Halbach, and Skare and Engquist, and mercury levels in urine, plasma, and scalp hair. Information about other covariates will be collected including parental intelligence, socioeconomic status, medical and dental information, other neurotoxic exposures such as maternal smoking and alcohol consumption during pregnancy, and child blood lead level. Blood samples will be collected at baseline and six months to measure inorganic mercury and other heavy metals that may confound neuropsychological test results. Four urinary mercury measurements will be collected during the first year, and then annually. Neuropsychological tests, hair sampling, and measurement of amalgam release rates will be conducted at baseline and annually. A subject retention rate of 75% at the end of five-years is anticipated.